A young gym enthusiast collapsed and died after eating a cereal bar after a workout – because exercise brought on a severe allergic reaction to nuts.

Maisie Durant is believed to have suffered from a rare condition known as exercise-induced anaphylaxis, where physical exercise sparks a severe and potentially fatal allergic reaction.

The 21-year-old had always had a nut allergy, but controlled her mild symptoms with over the counter medicine.

After going to the gym she picked up a cereal bar to take home to eat, without realising it contained nuts - to which she was allergic.

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Maisie Durant, 21, collapsed and died after eating a cereal bar she didn't realise contained nuts - to which she was allergic. It is believed she suffered from a rare condition known as exercise-induced anaphylaxis, where physical activity sparks a fatal allergic reaction

Miss Durant had been diagnosed with a nut allergy and asthma as a child, but had controlled the allergy using over the counter medicine Piriton. Before her death, she had never had a severe allergic reaction before. She is pictured, left and right, before she died

Miss Durant, a Royal Holloway University graduate, did not know exercise can worsen nut allergies because it causes the blood to pump faster around the body. She is pictured, left and right, during her university years

When Miss Durant got home she collapsed, was rushed to hospital in an ambulance, but died in front of her devastated father hours later.

Experts say exercising can worsen the symptoms of nut allergies because it causes the gut to become 'leaky'.

This allows the allergens to be passed into the blood faster, and blood to be pumped around the body more quickly.

HOW CAN EXERCISE TRIGGER AN ALLERGIC REACTION?

During anaphylaxis, the body’s immune system over-reacts to an allergen, triggering the release of the chemical histamine, which can affect the airways and mucous membranes (lips and eyes) and cardiovascular system.

Dr Stephen Till, a consultant in adult allergy at Guy’s and St Thomas’ NHS Foundation Trust in London said this allergic shock can be made worse by exercise.

He said: ‘The exercise involved can be moderate - one patient suffered anaphylaxis when pushing her baby’s pram up a hill - and in many different forms, from labouring on a building site or dancing in a nightclub to taking a walk or going for a run.'

But it’s thought that the more the person exerts themselves, the stronger the reaction.

Dr Till added: ‘One patient, a triathlete, reacted to something she had eaten the night before a race.’

Why exercise should spark anaphylaxis is unknown, but there are theories that it increases the permeability of the gut, changes the way the blood vessels react to allergens or has an effect on the body’s neuroendocrine cells (specialised nerve cells that produce hormones such as adrenaline).

Heat, alcohol, anxiety and mental stress are also thought to exacerbate the problem.

He said: 'She was grabbing for her pills for the nut reaction and then it became clear it was a shed load more serious,' he said.

'I'm not sure anyone can possibly understand what it's like to watch that take place.'

He called 999 and paramedics arrived but she died in hospital hours later.

'It was horrible,' he said.

'The health service did a great job but it was all too late.

'Now I want to warn people about exercise and allergies.'

He said Miss Durant had suffered from a nut allergy since she was a young girl, although no one realised the severity of it.

She took over-the-counter Piriton tablets to control it, but was not prescribed an EpiPen as she had no history of severe allergic reactions.

Mr Durant said his daughter was a bright, fun and generous girl.

He said: 'She was fun-loving and the centre of party attention, but also blossoming out for her own career.

'If she walked into a room full of strangers, she could handle that.

'She was also very bright. She liked having fun but there was also a serious, generous side to her.

'If she wanted something, she worked for it.'

Her family have set up a JustGiving page, and since Miss Durant's death, donations have flooded in.

So far more than £45,000 has been pledged to The Anaphylaxis Campaign, from both those who knew and loved Miss Durant, and strangers who have been touched by her story.

Her father said: 'Some people who we don't know have been immensely generous , just because they've got a son or daughter who has come close to the same fate as Maisie.

'We want to help protect other people from this.'

Her funeral took place on February 4 in Chiswick, and colleagues from her job at recruitment company Reed organised a minute's applause in tribute to her.

When she was 16, Miss Durant became an organ donor, so after her death she went on to save the lives of three others – including two kidney failure patients who've been on dialysis for five years.

While her family have not been told the names of the people who have received her organs, they have been informed that all three are currently in good health.

'That gives you a great indication of what Maisie was like,' Mr Durant said.

Mr Durant said his daughter was bright, generous and confident. He said: 'She was fun-loving and the centre of party attention, but also blossoming out for her own career. She is pictured (left), with her friend Lillie (right), before her death

Miss Durant's colleagues from her job at recruitment company Reed organised a minute's applause in tribute to her (pictured)

'There was a fun side to her but there was also a serious and generous side to her.

'She had a boyfriend, Chesney, and they were planning to go to Berlin together.

'Life was turning into quite an exciting place for her. She had a magnificent mix of focus, fun and heart.'

A spokesperson for the Anaphylaxis Campaign said: 'When someone goes into anaphylactic shock, we recommend they lay down, put their feet up, and don't walk around or do any type of movement, because this pumps the blood around the body, so Maisie exercising could've played a part.'